This invention generally relates to optical receivers, and more specifically, to transimpedance circuits used in optical receivers.
Optical receivers are used in a variety of devices such as photodetectors, optical detectors and optical sensors, to convert light to an electric current or voltage. An optical communication receiver starts with a photodiode, the device that converts input light intensity into a proportional electrical current. Typical values of the current are in the range of tens of microamperes, but can be smaller or larger, depending on the application. In order to be useful for the digital processing on the receiving side, this current has to be converted into the voltage domain and amplified. This function is typically performed by a transimpedance amplifier (TIA), followed by a limiting amplifier (LA). After the LA, the signal can be sampled (sliced) in a clocked decision circuit (latch). This completes the optical receiver function at the physical level.
The TIAs must provide sufficient bandwidth, sensitivity, dynamic range, high gain and low noise to achieve good system performance. In a conventional TIA receiver though, the requirements for high gain and low noise are in direct conflict with the requirement of high bandwidth.
One standard approach to increase the bandwidth of the receiver is to place a peaking amplifier immediately after the TIA. While overall bandwidth of the receiver can be increased in this manner, the main drawback of the peaking amplifier is that it will amplify high frequency noise, significantly degrading the input referred current noise and, as a result, dramatically reducing the sensitivity of the receiver.